fix: dnd not working in initial doc (#12519)

Fixes [BS-3152](https://linear.app/affine-design/issue/BS-3152/)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **Bug Fixes**
  - Improved widget stability by removing the event listener that caused the widget to hide on block updates, leaving only viewport changes to trigger hiding.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/widgets/drag-handle/src/watchers/page-watcher.ts

@@ -12,10 +12,6 @@ export class PageWatcher {
   watch() {
     const { disposables } = this.widget;
 
-    disposables.add(
-      this.widget.store.slots.blockUpdated.subscribe(() => this.widget.hide())
-    );
-
     disposables.add(
       this.pageViewportService.subscribe(() => {
         this.widget.hide();